At present the genus Blastomyces contains two species, Blastomyces dermatitidis and Blastomyces gilchristi, which are morphologically identical but distinguishable by sequence analysis of the ITS region (Brown et al. 2013). B. dermatitidis lives in soil and in association with decaying organic matter such as leaves and wood. It is the causal agent of blastomycosis a chronic granulomatous and suppurative disease, having a primary pulmonary stage that is frequently followed by dissemination to other body sites, typically the skin and bone. Although the disease was long thought to be restricted to the North American continent, in recent years autochthonous cases have been diagnosed in Africa, Asia and Europe.
WARNING: RG-3 organism. Cultures of B. dermatitidis represent a biohazard to laboratory personnel and must be handled in a Class II Biological Safety Cabinet (BSCII).
Morphological Description: Colonies at 25C have variable morphology and growth rate. They may grow rapidly, producing a fluffy white mycelium or slowly as glabrous, tan, nonsporulating colonies. Growth and sporulation may be enhanced by yeast extract. Most strains become pleomorphic with age. Microscopically, hyaline, ovoid to pyriform, one-celled, smooth-walled conidia (2-10 µm in diameter) of the Chrysosporium type, are borne on short lateral or terminal hyphal branches.
Colonies on blood agar at 37C are wrinkled and folded, glabrous and yeast-like. Microscopically, the organism produces the characteristic yeast phase seen in tissue pathology; ie. B. dermatitidis is a dimorphic fungus.
Comment: In the past, conversion from the mould form to the yeast form was necessary to positively identify this dimorphic pathogen from species of Chrysosporium or Sepedonium. However, culture identification by exoantigen test and/or molecular methods is now preferred to minimise manipulation of the fungus.
Key Features: Clinical history, tissue pathology, culture identification by positive exoantigen test and/or by molecular methods.
Tissue section showing large, broad-base, unipolar budding yeast-like cells and culture of Blastomyces dermatitidis.
Histopathology: Blastomyces dermatitidis tissue sections show large, broad-based, unipolar budding yeast-like cells, which may vary in size from 8-15 µm, with some larger forms up to 30 µm in diameter. Tissue sections need to be stained by Grocott’s methenamine silver method to clearly see the yeast-like cells, which are often difficult to observe in H&E preparations.
Molecular Diagnostics: A DNA probe assay (AccuProbe, Gen-Probe, Inc., San Diego, CA) for identification of B. dermatitidis in clinical isolates is available (Scalarone et al. 1992 and Padhye et al. 1994b). However this has limited application as it can be used only with pure cultures of B. dermatitidis (yeast or mould) (Sidamonidze et al. 2012). Several conventional PCR assays have been developed for the identification of B. dermatitidis from clinical specimens (Bialek et al. 2003) and soil (Burgess et al. 2006). Sidamonidze et al. (2012) developed a real-time PCR targeting the BAD1 (formerly known as WI-1) gene for the identification of B. dermatitidis in culture and tissue and Morjaria et al. (2015) used rDNA sequencing for identification from paraffin embedded tissue.
References: McGinnis (1980), Chandler et al. (1980), Kaufman and Standard (1987), Rippon (1988), Brown et al. (2013).
Rippon, J.W. 1988. Medical Mycology. 3rd Edition. W.B. Saunders Co., Philadelphia, USA.